Conventionally, practical use has been widely made of a core drill which uses a cylindrical core body having a drilling blade at a lower end edge thereof to drill a large-diameter hole.
A core drill of this type comprises a core body and a base body. The base body having a shank at an upper part thereof has a attaching portion at a lower part thereof, a core body attaching portion with a shoulder portion having a step portion around the attaching portion, engagement grooves situated at plural locations on the outer periphery of the core body attaching portion, each engagement groove comprising a longitudinal groove portion and a transverse groove portion continuous therewith, which is concave in section and appears like a hook when viewed from outside, and a restraining ball biased to protrude into the transverse groove portion of each engagement groove by a compression spring thereby holding the engagement projection at an engagement terminal point in order to keep the coupling to the core body. On the other hand, the core body has the engagement projection corresponding to each engagement groove, the engagement projection extending inwardly from an inner peripheral surface of an upper end portion of the cylindrical core body having a drilling blade at a lower end edge thereof.
The attaching the core body to the base body is achieved by attaching the upper end portion of the core body to the core body attaching portion of the base body to cause the engagement projections on the core body side to engage the engagement grooves on the base body side with the restraining balls being caused to pass through the engagement projections against the biasing force of the compression spring.
Such a prior art is described in Japanese Utility Model Examined Gazette No. HEI 7-39527 for example.
The core drill of the aforementioned construction allows the core body to be attached to or detached from the base body easily and hence is excellent in that, for example, when it is employed as a cutting tool of a drilling machine such as an electric drill, the two are separated from each other to allow masses resulting from cutting, which are left within the core body after drilling of concrete, stone or the like, to be removed easily.
Though the aforementioned core drill of the construction allowing attaching and detaching the core body to or from the base body is excellent in that the state of coupling between the base body and the core body is stably maintained during a drilling operation by the arrangement of the hook-shaped engagement grooves and the restraining balls if the core drill drills an object at constant revolutions, the core drill has the following inconvenience if it is used as a cutting tool of a “rotary hammer drill”, a sort of drilling machine, to drill concrete, stone or the like. That is, since the “rotary hammer drill” imparts periodical axial strikes together with rotation to the core body side from the drilling machine side, impactive forces are exerted on the cutting tool side during the drilling operation to cause the coupling between the base body and the core body to rattle thereby making the coupling state thereof unstable.
Recently, electric drills of the type having a braking mechanism such as to rapidly stop rotation have been commercially available. When the core drill is used with an electric drill of this type, rapid braking during rotation makes the state of coupling between the base body and the core body unstable due to rotational inertia.
The present invention has been made in view of the aforementioned present circumstances, and it is an object of the present invention to provide a core body locking device of a core drill which is free of any one of the aforementioned inconveniences as well as a core body for use therein.